Te Lucis ante Terminum is an Ambrosian hymn from the early middle-ages. In monasteries it was sung at the Compline at the end of each day.
Several different plainsongs of this hymn do exist. Marcel Dupré used one of them for his organmusic, published in the book ‘Le tombeau de Titelouze’.
The words of the first is couplet are mentioned in the score:
Te lucis ante términum,
Rerum Creátor, póscimus
Ut pro tua cleméntia
Sis praesul et custódia.
To Thee, before the close of day
Creator of the world, we pray
that with Thy wonted favor, Thou
wouldst be our Guard and Keeper now.
Until now I could not find the origin of the plainsong used by Marcel Dupré.